Check Your Local Exhaust Ventilation System

May 5, 2000
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Local exhaust ventilation systems (LEVS) are meant to capture airborne chemicals at the source of generation and remove contaminants from the work area. When a LEVS does its job your workers are healthy and productive. But often LEVS don't do their job. Why?

First, look at the number of components that must be working to ensure a LEVS is functioning properly. A LEVS consists of:

  • hoods for capturing the contaminant

  • ducts for transporting the contaminant

  • air cleaner for removing contaminants from the air stream

  • fan to create airflow in the system

  • stack to discharge the air outside the workplace.


So, why don't some LEVS do the job?

In my experience, a LEVS fails to do the job because the system or one or more of its components were either not designed for the job, or were not installed properly, used properly or maintained properly. To get your LEVS system back on the job you will need to address these areas of possible failure.

Design for success

LEVS that work are usually the ones that were designed specifically to meet the requirements of the job. In order to design an effective LEVS first define what you want it to do.

Know the chemical

To design a LEVS you must know all about the chemical you are dealing with and how it is used. For example, know the:
  • physical state of the chemical (Is it a dust, mist, fume, gas or vapor?),
  • chemical's toxicity and applicable exposure limits,
  • physical properties of the chemical (Vapor pressure, boiling point, flash point),
  • routes of worker exposure -- inhalation, ingestion, skin contact,
  • how, where and when the chemical is used,
  • and how the worker does their job.

Hood design -- A well-designed hood is the most important component of an effective LEVS. The hood must be positioned so that it does not pull contaminated air through the worker's breathing zone. It should be easy to use and not interfere with the job that the worker is trying to do. It should be positioned as close to the point of contaminant generation as possible. The further it is from the point where the chemical is released into the air, the more airflow is required to capture the contaminant.

Air volume and capture velocity -- The air volume (cubic feet per minute) that must be exhausted by LEVS is determined by the type of hood, the distance of the hood from the source of the contaminant and the velocity needed to capture the contaminant (Capture Velocity). Capture velocity for a hood is determined by the properties of the chemical and how it is being used. Examples of capture velocities are shown in Table 1.

Make up air -- Air will only be exhausted to the extent that air enters the workplace. If you don't provide make up air in the amount at least equal to the amount of air being exhausted, your LEVS will not work properly and the workplace will be very drafty, doors will be difficult to open, and furnaces, heaters or other combustion equipment may back draft.

Transport velocity and duct size -- Once a contaminant is captured by the hood it moves into the duct system. The velocity in the duct must be sufficient to transport the contaminant through the LEVS. The velocity in the duct necessary to carry the contaminant through the system is referred to as the transport velocity. The heavier the contaminant the higher the velocity needed for transport. Some examples of transport velocities for different contaminants are shown in Table 2. Once you know the airflow volume and transport velocity needed for a LEVS, the duct size can be calculated using the formula shown in Table 3.

Streamline airflow -- LEVS should always use round ducts, because airflow is more uniform and streamlined, which makes the system more efficient and provides better transport for contaminants. The duct runs should be as straight as possible; curves should be smooth and gradual; and an elbow should have a radius of 2 to 2.5 times the duct diameter. Branch entries into the main duct should be at an angle of 45?; there should be no 90? entries. All changes in size should be smooth and gradual.

Stacks -- A stack should discharge contaminated air vertically upward and away from the building. Stacks should be located as far from air intake units as possible to prevent reintroduction of contaminated air into the building. The top of the stack should be 1.3 to 2 times the building height above the ground. Avoid exhausting air out of the sides of buildings. The pressure of prevailing winds blowing into the exhaust can severely affect the performance of the LEVS.

Fan selection -- The fan you select for your LEVS should be based on the needs of the system. It should not only deliver the volume of air (in cubic feet per minute) necessary to capture the contaminant but be able to do so against the resistance to airflow in the system. The resistance to airflow is measured in inches of water and is usually referred to as static pressure losses. Static pressure losses in a LEVS are determined by the:

  • size of the duct,
  • roughness of the duct material,
  • number and type of elbows, entries, and changes in size,
  • type of air cleaner,
  • type of hood,
  • volume of air flowing in the system, and
  • stack design.

It should be clear from this list that a fan cannot possibly be selected successfully until the system has been designed.

System installation -- Insist that you get a system installed as designed, with round ducts and smooth streamlined airflow. Since a rough duct increases static pressure losses and requires a larger fan, you should keep the use of a flexible duct, which is very rough, to a minimum. Use a flexible duct only where you need flexibility and use as little of it as possible.

Fans will operate more efficiently if they are installed with a length of straight duct entering and leaving the fan. A rule of thumb is to provide a straight run of duct at least six duct diameters long on the entrance side of the fan and at least two duct diameters long on the exit side. After installing the system, measure to ensure that the LEVS delivers the airflow volume and velocity that is needed to do the job.

Keep it user-friendly

A LEVS should be easy for the worker to use. If the worker does not understand the reason for the system, or does not know how to use the system, or if it interferes with job performance, the LEVS is not going to work. So involve the worker in the entire process of designing, installing and starting up a new LEVS.

Maintain It

A LEVS is a mechanical system that must be maintained or it will fail over time. Some examples of failures that I have seen include:

  • Fan not turned on,

  • Fan turning in the wrong direction,

  • Fan belt broken,

  • Duct blocked from accumulated contaminant,

  • Holes in duct,

  • Hoods damaged or removed,

  • Lack of make up air.

Inspect all components of the LEVS system periodically to ensure it is operating properly. Scheduled maintenance should include: measurement of total airflow, duct velocity and capture velocity for comparison with baseline measurements taken at the time of installation.

Sources of Help

"Industrial Ventilation Manual" published by the American Conference of Governmental Industrial Hygienists (ACGIH). For other publications on ventilation and correspondence course on ventilation principles contact, ACGIH at 1330 Kemper Meadow Drive, Ste 600, Cincinnati, OH 45240 or call them at (513) 742-2020 or fax them at (513) 742-3355. Web site: www.acgih.org. OSHA's Technical Manual. Web site: www.oshaslc.gov/SLTC/ventilation/index.html.

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